Creating User Defined Function

A function definition always begins with the keyword def (short for define). The general syntax for creating a user defined function is:

def function_name([parameter1, parameter2, ...]):
    statement(s)
    [return value(s)]

Read the syntax with these rules in mind:

• The items enclosed in [ ] are parameters, and the brackets mean they are optional — a function may or may not have parameters. Likewise, a function may or may not return a value (the return line is optional too).
• The function header always ends with a colon (:).
• The function name must be unique, and the same rules that apply to naming identifiers in Python apply to naming functions.
• The body of the function is marked by indentation: any statement written outside the function's indentation is not part of the function.

A first user defined function

The task: write a user defined function that accepts two numbers from the user, calculates their sum and displays it. The requirements break down as — (1) accept 2 numbers from the user, (2) calculate their sum, (3) display the sum.

# Function to add two numbers and display their sum

# function definition
def addnum():
    fnum = int(input("Enter first number: "))
    snum = int(input("Enter second number: "))
    sum = fnum + snum
    print("The sum of", fnum, "and", snum, "is", sum)

# function call
addnum()

Notice that addnum() takes no parameters (the parentheses in the header are empty) and returns nothing — it reads its own inputs and prints its own result.

Calling the function …